This invention relates to loudspeakers and more particularly to sound reproducing devices capable of operating at below 100 Hz and commonly referred to as subwoofers, which attempt to reproduce sounds in the lowermost range of human hearing.
With few exceptions, all loudspeakers available today utilize a cone diaphragm driven by a movable voice coil, which is suspended between the pole pieces of a permanent magnet. Electrical energy conveyed to the voice coil causes the coil to reciprocate in a linear path and move the diaphragm. This type of speaker is commonly known as the permanent magnet dynamic type and generally has an efficiency of less than 5 percent and even less at low frequencies.
A long standing objective in high fidelity sound systems is to provide a speaker that will accurately reproduce low frequency sounds down to the lowermost limits of human hearing. Several problems have prevented the attainment of this objective. It has not been possible to produce sounds down to 20 Hz at sufficiently intense sound levels using conventional speaker design. Also, attempts to reproduce low frequency sounds typically result in excessive distortion, due to the non-linearity of the drive at low frequencies.
The maximum sound pressure level available at low frequencies is dependent upon the acoustic source strength, which is specified by the available area of the vibrating surface and the peak amplitude of vibration. Thus, available acoustic power is dependent upon the volume of air that is "pumped" by the diaphragm. To maintain a constant sound pressure level, each halving of the frequency requires a quadrupling of the peak to peak excursion.
In attempt to achieve accurate low frequency reproduction, conventional speakers have been provided with long voice coils and large magnets, diaphragms and enclosures, etc. There are, however, several limits in the design of such a speaker. First, there is a practical limit on magnet size, design and weight. Also, activation of the longer voice coil results in large power losses in the form of heat (I.sup.2 R losses). Possible thermal destruction of the coil imposes a limit on the power handling capacity of the speaker. Moreover, at low frequencies, a point is soon reached at which the driver ceases to operate in a linear fashion because the voice coil is driven out of the region of constant magnetic flux. All of such drives have a limited degree of excursion, which limits the available displacement of the diaphragm.
Another consideration is the relative insensitivity of the human ear to low frequencies, which in turn, requires such low frequencies to be produced at more intense levels to be heard. As illustrated in the Fletcher-Munson hearing sensitivity curves, the threshold of hearing is zero dB at 1000 Hz, but is 40 dB at 100 Hz and about 100 dB at 20 Hz. Since a change of 40 dB involves a corresponding power multiplication of 10,000, attainment of non-distorted sound frequencies in the region of 20 to 60 Hz and at high sound levels has not been practical using conventional apparatus and techniques. No other satisfactory solutions to the foregoing problems have been forthcoming, and low frequency response has been sacrificed with the use of small enclosures and the desire to produce a reasonable spectrum of wavelengths at an affordable price.
In the early stages of speaker development, several proposals were made to utilize a galvanometer-type drive having a rotary output to drive one or several sound radiating panels. Such devices are described in British Pat. Nos. 271,021, 270,421, 212,857, Austrian Pat. No. 126,717, and Japanese Pat. No. 11,384. The drives of all these devices, however, are all in the form of a single coil immersed between two poles of a permanent magnet, which seriously limits available excursion. Also, the available force decreases as the coil departs the field. There are also limits on magnitude of available peak force and power handling capacity, since the drives in these devices have the same or similar limitations as are found in conventional, permanent magnet speakers.
Until recently, there was very little need to reproduce intense levels of sound in the range of 20 to 60 Hz because available programming sources were incapable of recording such frequencies. With the advent of more dynamic recording techniques, however, the ability to produce such sounds without distortion has become a highly desirable objective in the industry.